The present invention relates to a transistor provided with a current detecting function for detecting the magnitude of a main current. It can be applied to various kinds of transistors including those of the bipolar, electrostatic induction and field effect types.
The present invention also relates to a transistor provided with a current detecting function, such as a main transistor through which main current flows and a sense transistor for detecting the current flowing through the main transistor, and more particularly to a transistor provided with a current detecting function designed to enable stable sense voltages to be obtained, which are not affected by temperature change.
In one conventional system, a power transistor through which a large amount of current must flow is provided with a current detecting function to prevent it from being damaged by excess current. An external resistance is connected between the main electrodes, such as the emitter and source electrodes, and the voltage drop across this resistance is measured. Thus, the magnitude of the main current is detected. This method, however, requires the addition of an external resistance, thus causing power loss.
In another conventional system, a transistor provided with a sense transistor is connected to the power transistor independently of the main transistor, and to the current detecting emitter terminal. In this system, a resistance (hereinafter referred to as sense resistance) must be located inside the transistor to detect the drop in sense voltage in the current flowing through the sense resistance. Materials such as aluminum, polysilicon, diffusion resistance and the like are used for this sense resistance.
When these materials are used, the sense resistance changes with temperature because they have their own temperature coefficients. Therefore, the sense voltage also changes with temperature.
This problem will be solved if the sense resistance is made independent of temperature change.
However, the resistance (or on-resistance) between the collector and the emitter of a transistor is inclined to change with temperature under operating conditions. Therefore, the step to solve this problem is not sufficient. In other words, the ratio of currents (main and sense currents) flowing through the main and sense transistors changes with temperature, thereby causing sense voltage to change with temperature.